1. Field of Invention
This invention relates generally to submicron particle filter units of the Hepa type, and more particularly to a unit whose membrane is fabricated of a continuous porous metal sheet accordion-folded to define a uniform series of pleats forming a media pack that is held within a frame.
2. Status of Prior Art
My above-identified copending application discloses a laminar flow system for drying microelectronic, optical and other parts that in the course of their processing are rendered wet with solvents or other liquids, which if not fully evaporated from the parts will contaminate them and impair their quality. The system includes an open-ended drying chamber having intermediate its sides a work zone which the parts to be dried are supported for exposure to a gas stream. One end of the chamber defines a gas inlet to admit the gas stream which sweeps the entire chamber. Adjoining the drying chamber at its inlet is a pressure chamber in which a submicron particle filter unit of the Hepa type producing a relatively low pressure drop is disposed to cover the gas inlet.
Pressurized gas fed into the pressure chamber and heated to an elevated temperature imposes a distributed pressure on the entry face of the Hepa filter unit and passes therethrough to yield at its exit face a laminar stream of hot gas that subjects the parts in the work zone to a substantially uniform drying action, so that all parts being treated are fully dried.
A conventional Hepa filter unit is mainly composed of a continuous sheet of non-woven, non-combustible microglass fibers having high tensile strength and water-repellant properties. This non-woven sheet is accordion-folded over narrow fluted corrugated aluminum separators supported within a frame and bonded thereto by sealants. Also provided are gaskets on either face of the frame to form a leak-proof seal at the air exit and entry faces of the filter unit.
A commercial Hepa filter unit of this type is available commercially under the trademark MICROFLO from the High Efficiency Filter Corp. of Eatontown, N.J. While a standard Hepa filter unit can withstand temperatures as high as 220.degree. F., in the drying system disclosed in my copending application in which air or nitrogen gas is heated to as high as 200.degree. C. in order to promote rapid evaporation, it is essential that the Hepa filter unit be capable of functioning at elevated temperatures. To this end, the unit disclosed in my copending application is provided with sealants, gaskets and frames fabricated of material capable of withstanding the elevated temperatures to which the unit is subjected.
The MICROFLO Hepa filter unit is constituted by a filter media pack constructed from a continuous sheet of non-woven microglass fibers bonded with acrylic resin. The media pack is permanently sealed within a cell made of particle board or other material by rubber-based adhesive sealants.
It has been found that when a Hepa type filter unit, whose membrane is formed of a pleated sheet of non-woven fiberglass material, is incorporated in a laminar flow drying system of the type disclosed in my copending application, because of the elevated temperatures involved and the chemical nature of the bonding and sealing agents, gases are evolved from these agents. The resultant out-gassing in some instances gives rise to unacceptable levels of contamination in the work being dried, particularly in the case of microelectronic parts.
The use of Hepa type filter units is by no means limited to parts drying systems, for these filters are widely used in the design of so-called "clean rooms" to create a particle-free, pure environment suitable for fabricating micro-circuits and other critical components, as well as in the production of pharmaceuticals and biologicals. Hepa type filter units are also commonly employed in association with laminar flow clean hoods and in other equipment dictating a particle-free, critically-clean environment.
For many applications, commercially-available Hepa filter units whose membrane is formed of a pleated fiberglass sheet perform in a satisfactory manner, and to the extent that out-gassing is experienced by reason of sealants and bonding agents included in these units, it is tolerable. But as the technology of miniaturization continues to advance and as micro devices in the course of their manufacture become increasingly intolerant of even slight traces of contaminants in the environment, then even minimal amounts of out-gassing constituents which become entrained in the air or nitrogen stream from the Hepa filter unit have been found to be unacceptable.
Moreover, in certain applications where the nature of the equipment is such as to dictate a heated gas environment, such as in an air or nitrogen drying oven, the out-gassing propensity of sealants and bonding agents included in known types of Hepa filters becomes greater, and the risk of contaminating the controlled environment which it serves is thereby increased.